Technical Field
The present disclosure relates to a micro-electro-mechanical pressure sensor and methods of forming same.
Description of the Related Art
As is known, sensors including micromechanical structures made, at least in part, of semiconductor materials and with MEMS (Micro-Electro-Mechanical Systems) technology are increasingly used, due to their advantageous characteristics of small dimensions, low manufacturing costs, and flexibility.
A MEMS sensor generally comprises a micro-electro-mechanical structure that transduces a physical or mechanical quantity to be detected into an electrical quantity (for example, correlated to a capacitive variation) and an electronic reading circuit, usually an ASIC (Application Specific Integrated Circuit), which executes processing operations (among which amplification and filtering) of the electrical quantity and outputs an electrical signal, either analog (such as a voltage), or digital (such a PDM—Pulse Density Modulation signal). The electrical signal, in case further processed by an electronic interface circuit, is then made available to an external electronic system, for example a microprocessor control circuit of an electronic apparatus incorporating the sensor.
In micro-electro-mechanical structures, detection of the desired physical or mechanical quantity is obtained by virtue of a membrane formed in or on a semiconductor chip and suspended over a cavity. The membrane may be arranged facing the external environment or be in communication therewith via a fluidic path.
Italian patent application TO2013A000540 filed on 28 Jun. 2013 (which corresponds to U.S. Pat. Pub. No. 2015/001651) describes, for example, an MEMS device, wherein a sensitive part of the device including the membrane is separated from the rest of the chip and supported by springs. The springs decouple the sensitive part from the rest of the chip and absorb the packaging stress, without transferring it to the sensitive part.
FIG. 1 shows in a simplified way an MEMS sensor 1 formed in a chip 10 of semiconductor material, such as silicon, subject of patent application TO2013A000540 referred to above, here illustrated as a pressure sensor. A cap 11 is fixed to a first face 10A of the chip 10 through first spacers 22, and a closing region 12 is fixed to a second face 10B of the chip 10 via second spacers 26.
The chip 10 comprises a suspended region 13 separated from a peripheral portion 18 of the chip 10 through a trench 14. Elastic elements (also referred to as springs 15) support the suspended region 13 and connect it mechanically to the peripheral portion 18. The suspended region 13 forms a membrane 19 delimited at the bottom by a buried cavity 16.
In the pressure sensor of FIG. 1, the membrane is obtained by epitaxial growth in a deoxidizing environment above the buried cavity 16. Consequently, a certain amount of gas (generally, hydrogen) remains within the buried cavity 16. As temperature varies, the trapped gas expands or contracts and may generate a spurious pressure against the membrane. This may cause a reading error, generating an offset.